.container{
	position:relative;
	width: 400px;
	height: 200px;
	border: 1px solid green;
	/*位置*/
	text-align: center;
	margin-top:100px;
	margin-left: auto;
	margin-right: auto;
}
.ball{
	position: absolute;
	width: 15px;
	height: 15px;
	top:10px;
	left:10px;
}